Salt hydrate-based phase change thermal energy storage and encapsulation thereof

ABSTRACT

Among other things, the present disclosure relates to phase change material (PCM) composites composed of an PCM mixed with a nucleating agent contained within the pores of a graphite matrix and/or a hydrogel. The process to create these PCM composites includes coating the surface of graphite with a surfactant, compressing the graphite to form a matrix, then filling the graphite matrix with the PCM.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication No. 63/033,878, filed on Jun. 3, 2020, the contents of whichare incorporated herein by reference in their entirety.

CONTRACTUAL ORIGIN

The United States Government has rights in this invention under ContractNo. DE-AC36-08G028308 between the United States Department of Energy andAlliance for Sustainable Energy, LLC, the Manager and Operator of theNational Renewable Energy Laboratory.

SUMMARY

An aspect of the present disclosure is a composition including agraphite matrix comprising an expanded graphite having a plurality ofpores defining a pore volume, a surfactant having a first end and asecond end, a mixture comprising a phase change material and anucleating agent positioned within between about 40% and about 95% ofthe pore volume, in which the first end of the surfactant is bonded tothe expanded graphite, the second end of the surfactant is bonded to thephase change material, the surfactant is present in a mass ratio of thesurfactant to the expanded graphite between about 1:100 and about 5:100,and the nucleating agent is present in the mixture at a concentrationbetween greater than zero weight percent (wt %) and less than about 6.0wt %. In some embodiments, the pore volume is between about 60% andabout 95% of a total volume of the graphite matrix as defined by thepore volume plus a volume of the expanded graphite present in thegraphite matrix. In some embodiments, the surfactant includes at leastone of octyl phenol ethoxylate (C₁₄H₂₂O(C₂H₄O)_(n) where n=9-10)(TX-100) or polyethylene glycol tert-octylphenyl ether(C₁₄H₂₂O(C₂H₄O)_(n) where n=7-8) (TX-105). In some embodiments, themixture further comprises a hydrogel comprising at least one ofpoly(acrylamide-co-acrylic acid) (PAAAM), poly(acrylamide-co-sodiumacrylate), or alginate. In some embodiments, the phase change materialcomprises a salt hydrate. In some embodiments, the salt hydrate includesat least one of calcium chloride hexahydrate (CaCl₂).6H₂O), calciumbromide hexahydrate (CaBr₂.6H₂O), disodium sulfate decahydrate(Na₂SO₄.10H₂O), disodium phosphate dodecahydrate (Na₂HPO₄.12H₂O), zincnitrate hexahydrate (Zn(NO₃)₂.6H₂O), magnesium chloride hexahydrate(MgCl₂.6H₂O), magnesium nitrate hexahydrate (Mg(NO₃)₂.6H₂O), or lithiumnitrate trihydrate (LiNO₃.2H₂O). In some embodiments, the nucleatingagent includes at least one of strontium chloride hexahydrate(SrCl₂.6H₂O), strontium bromide hexahydrate (SrBr₂.6H₂O), strontiumiodide hexahydrate (SrI₂.6H₂O), barium iodide hexahydrate (BaI₂.6H₂O),barium chloride hexahydrate (BaCl₂.6H₂O), barium chloride (BaCl₂),barium carbonate (BaCO₃), strontium carbonate (SrCO₃), barium fluoride(BaF₂), strontium fluoride (SrF₂), barium hydrofluoride (Ba(HF₂)),barium oxide (BaO), barium hydroxide (Ba(OH)₂)), barium sulfate (BaSO₄),or strontium hydroxide (Sr(OH)₂).

An aspect of the present disclosure is a method including, in order,heating an intercalated graphite to a temperature between about 200° C.and about 750° C. resulting in an expanded graphite, coating theexpanded graphite with a surfactant having a first end and a second endto form a wetted graphite, compressing the wetted graphite to form agraphite matrix having a plurality of pores defining a pore volume, andfilling between about 40% and about 95% of the pore volume with amixture including a phase change material and a nucleating agentresulting in an energy storage material, in which the surfactant ispresent in a mass ratio of the surfactant to the expanded graphitebetween about 1:100 and about 5:100, the first end of the surfactant isbonded to the expanded graphite and the second end of the surfactant isbonded to the phase change material, and the nucleating agent is presentin the mixture at a concentration between greater than zero wt % andless than about 6.0 wt %. In some embodiments, the pore volume isbetween about 60% and about 95% of a total volume of the graphite matrixas defined by the pore volume plus a volume of the expanded graphitepresent in the graphite matrix. In some embodiments, the heatingincludes placing the intercalated graphite in a furnace for a period oftime in which the furnace is operated at a temperature of between about200° C. and 750° C. In some embodiments, the period of time is betweenabout one (1) minute and about ten (10) minutes. In some embodiments,the coating includes submerging the expanded graphite in a solutionincluding the surfactant. In some embodiments, the surfactant includesat least one of octyl phenol ethoxylate (C₁₄H₂₂O(C₂H₄O)_(n) wheren=9-10) or polyethylene glycol tert-octylphenyl ether(C₁₄H₂₂O(C₂H₄O)_(n) where n=7-8). In some embodiments, the compressingincludes placing the wetted graphite in a hydraulic press having apellet die, and pressing the pellet die on the wetted graphite resultingin the graphite matrix. In some embodiments, the filling includesmelting the phase change material and the nucleating agent to form themixture and submerging the graphite matrix in the mixture resulting in aslurry. In some embodiments, the filling also includes performing vacuumfiltration on the slurry during the submerging, resulting in the energystorage material. In some embodiments, a hydrogel including at least oneof poly(acrylamide-co-acrylic acid) (PAAAM), poly(acrylamide-co-sodiumacrylate), or alginate is added to the mixture prior to the filling. Insome embodiments, the phase change material includes a salt hydrate. Insome embodiments, the salt hydrate includes at least one of calciumchloride hexahydrate (CaCl₂).6H₂O), calcium bromide hexahydrate(CaBr₂.6H₂O), disodium sulfate decahydrate (Na₂SO₄.10H₂O), disodiumphosphate dodecahydrate (Na₂HPO₄.12H₂O), zinc nitrate hexahydrate(Zn(NO₃)₂.6H₂O), magnesium chloride hexahydrate (MgCl₂.6H₂O), magnesiumnitrate hexahydrate (Mg(NO₃)₂.6H₂O), or lithium nitrate trihydrate(LiNO₃.2H₂O). In some embodiments, the nucleating agent includes atleast one of strontium chloride hexahydrate (SrCl₂.6H₂O), strontiumbromide hexahydrate (SrBr₂.6H₂O), strontium iodide hexahydrate(SrI₂.6H₂O), barium iodide hexahydrate (BaI₂.6H₂O), barium chloridehexahydrate (BaCl₂.6H₂O), barium chloride (BaCl₂), barium carbonate(BaCO₃), strontium carbonate (SrCO₃), barium fluoride (BaF₂), strontiumfluoride (SrF₂), barium hydrofluoride (Ba(HF₂)), barium oxide (BaO),barium hydroxide (Ba(OH)₂)), barium sulfate (BaSO₄), or strontiumhydroxide (Sr(OH)₂).

BACKGROUND

Thermal energy storage (TES) systems are technologies that promise toimprove the energy efficiencies of systems such as in buildingenvelopes, appliances, and heating, ventilation, and air conditioning(HVAC) systems. Phase change materials (PCMs) are of particular interestas TES systems for HVAC applications due to their high energy storagedensity and ability to store energy at constant (or near constant)temperature. Low temperature (between about 5° C. and about 50° C.) TESmaterials are of particular interest for buildings energy applications.

Hydrocarbon derived alkane paraffins are currently the most widely usedPCMs for building applications, however, their widespread adoption inbuildings has been hindered by their high cost, low volumetric energycapacities, and flammability. Inorganic PCMs show great promise due totheir high volumetric energy density, abundance in nature and low cost.However, technical materials challenges facing with these materialsinclude excessive supercooling (due to, for example, poor nucleationproperties), incongruent melting/phase separation leading to short cyclelife, and low thermal conductivity leading to slow charging/discharging.Thus, there remains a need for efficient latent heat TES systems forbuildings, HVAC, and other applications.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments are illustrated in referenced figures of the drawings.It is intended that the embodiments and figures disclosed herein are tobe considered illustrative rather than limiting.

FIG. 1 illustrates a method of making a phase change material (PCM)composite, according to some aspects of the present disclosure.

FIG. 2 illustrates scanning electron microscope (SEM) images ofintercalated graphite flakes, according to some aspects of the presentdisclosure.

FIG. 3 illustrates intercalated graphite, according to some aspects ofthe present disclosure.

FIG. 4 illustrates expanded graphite (EG) after being heated, accordingto some aspects of the present disclosure.

FIG. 5 illustrates SEM images of EG after being heated, according tosome aspects of the present disclosure.

FIG. 6 illustrates the percent total volume of the graphite matrixfilled with a PCM compared to the porosity of graphite matrix, accordingto some aspects of the present disclosure.

FIG. 7 illustrates contact angle measurements of a drop of PCM on thesurface of a graphite matrix A) which had not been coated withsurfactant, B) which was coated with surfactant after being compressed,and C) which had been coated with surfactant prior to being compressed,according to some aspects of the present disclosure.

FIG. 8 illustrates EG being soaked in a solution containing asurfactant, according to some aspects of the present disclosure.

FIG. 9 illustrates a graphite matrix, formed by compressing EG which hadbeen coated with surfactant, according to some aspects of the presentdisclosure.

FIG. 10 illustrates SEM images of graphite matrices having A) 62%porosity, B) 73% porosity, C) 83% porosity, and D) 91% porosity,according to some aspects of the present disclosure.

FIG. 11 illustrates graphite matrix coated in surfactant A) soaking in amixture of PCM and nucleating agent, and B) undergoing vacuum soaking,according to some aspects of the present disclosure.

FIG. 12 illustrates a graphite matrix which is substantially filled withPCM, according to some aspects of the present disclosure.

FIG. 13 illustrates SEM images of a graphite matrix coated withsurfactant; A) coated with surfactant after being compressed and havinga porosity of 84%, and B) coated with surfactant prior to beingcompressed and having a porosity of 83%, according to some aspects ofthe present disclosure.

FIG. 14 illustrates SEM images of a graphite matrix substantially filledwith A) a PCM of calcium chloride hexahydrate (CaCl₂).6H₂O) (CCH) bysoaking, B) a PCM of CCH by pre-compression vacuum filtration, C) a PCMof disodium phosphate dodecahydrate (Na₂HPO₄.12H₂O) (DHPD) by soaking,and D) a PCM of DHPD by pre-compression vacuum filtration, according tosome aspects of the present disclosure.

FIG. 15 illustrates the relationship between the PCM:EG mass ratio andthe bulk density, according to some aspects of the present disclosure.

FIG. 16 illustrates the PCM accumulation within the graphite matrix overtime, according to some aspects of the present disclosure.

FIG. 17 illustrates the relationship between heat flow and temperaturefor a mixture of PCM of CCH and 1, 2, and 5% of a nucleating agent ofstrontium chloride hexahydrate (SrCl₂.6H₂O) (SCH) (subjected to 4cycles), according to some aspects of the present disclosure.

FIG. 18 illustrates cooling curves of pure PCM and graphite matricesfilled with PCM, over four cycles, according to some aspects of thepresent disclosure.

FIG. 19 illustrates cooling curves of pure PCM and PCM composites overfour cycles, according to some aspects of the present disclosure.

FIG. 20 illustrates enthalpy compared with temperature for A) a PCM ofCCH, and B) a PCM of DHPD, according to some aspects of the presentdisclosure.

DETAILED DESCRIPTION

The embodiments described herein should not necessarily be construed aslimited to addressing any of the particular problems or deficienciesdiscussed herein. References in the specification to “one embodiment”,“an embodiment”, “an example embodiment”, “some embodiments”, etc.,indicate that the embodiment described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is submitted that it iswithin the knowledge of one skilled in the art to affect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

As used herein the term “substantially” is used to indicate that exactvalues are not necessarily attainable. By way of example, one ofordinary skill in the art will understand that in some chemicalreactions 100% conversion of a reactant is possible, yet unlikely. Mostof a reactant may be converted to a product and conversion of thereactant may asymptotically approach 100% conversion. So, although froma practical perspective 100% of the reactant is converted, from atechnical perspective, a small and sometimes difficult to define amountremains. For this example of a chemical reactant, that amount may berelatively easily defined by the detection limits of the instrument usedto test for it. However, in many cases, this amount may not be easilydefined, hence the use of the term “substantially”. In some embodimentsof the present disclosure, the term “substantially” is defined asapproaching a specific numeric value or target to within 20%, 15%, 10%,5%, or within 1% of the value or target. In further embodiments of thepresent disclosure, the term “substantially” is defined as approaching aspecific numeric value or target to within 1%, 0.9%, 0.8%, 0.7%, 0.6%,0.5%, 0.4%, 0.3%, 0.2%, or 0.1% of the value or target.

As used herein, the term “about” is used to indicate that exact valuesare not necessarily attainable. Therefore, the term “about” is used toindicate this uncertainty limit. In some embodiments of the presentdisclosure, the term “about” is used to indicate an uncertainty limit ofless than or equal to ±20%, ±15%, ±10%, ±5%, or ±1% of a specificnumeric value or target. In some embodiments of the present disclosure,the term “about” is used to indicate an uncertainty limit of less thanor equal to ±1%, ±0.9%, ±0.8%, ±0.7%, ±0.6%, ±0.5%, ±0.4%, ±0.3%, ±0.2%,or ±0.1% of a specific numeric value or target.

Among other things, the present disclosure relates to phase changematerial (PCM) composites composed of a mixture that includes a PCM anda nucleating agent contained within a graphite matrix. As shown herein,in some embodiments of the present disclosure, a mixture may alsoinclude a hydrogel. The graphite matrix may be coated with a surfactantto connect the PCM to the graphite matrix. A process to create these PCMcomposites may include applying a surfactant to a surface of thegraphite (e.g., internal surface of an internal pore) resulting in awetted graphite, compressing the surfactant-coated graphite to form agraphite matrix (i.e., wetted graphite), then filling at least a portionof the pores of the graphite matrix volume with the PCM. This processmay result in a greater percentage of the graphite matrix internal porevolume being filled with PCM compared to traditional PCM preparationmethods. Additionally, the PCM may be mixed with a nucleating agentprior to filling the graphite matrix. The presence of a nucleating agentmay prevent supercooling, which is desirable because supercoolingreduces the energy storage capabilities of a PCM. A hydrogel may be usedin conjunction with or in place of the graphite matrix to contain (orencapsulate) the PCM. These PCM composites may be used in variousapplications, including for in building thermal energy storageapplications.

In some embodiments of the present disclosure, a PCM may be a salthydrate. Graphite is hydrophobic and non-polar, while salt hydrates arehydrophilic and polar. To address this, some embodiments of the presentdisclosure include coating the surface of the graphite with a surfactant(i.e., a surface-active agent) before compressing the graphite where thecompressing forms a matrix and substantially filling the pores of thematrix with PCM. Due to its hydrophobic end, the surfactant wets thesurface of the internal pores and provides a hydrophilic end capable ofhydrogen bonding to the PCM (e.g., salt hydrate). In this manner, thesurfactant acts as a “bridge” between the hydrophobic graphite and thehydrophilic PCM. This may allow for the more successful impregnation(i.e., filling) of the porous graphite matrix with salt hydrates (i.e.,encapsulation of the salt hydrate by the graphite matrix).

FIG. 1 illustrates an exemplary method 100 of making a PCM composite,according to some aspects of the present disclosure. The method 100includes, in order, heating 105 a graphite to form pores in thegraphite, coating 110 the graphite with a surfactant, compressing 115the graphite to form a graphite matrix (a structure in which othermaterials may be embedded), and filling 120 the graphite matrix with amixture of a PCM and a nucleating agent. The order of the method 100 isimportant, as coating 110 the graphite prior to compressing 115 thegraphite enables a maximum amount of the internal surface area of thegraphite to be coated with surfactant, which in turn maximizes thepenetration of the PCM into the matrix during the filling 120 becausethe surfactant connects the PCM to the graphite (i.e., the “bridge”described above).

As described in more detail below, a method 100 for making a PCMcomposite for energy storage may begin with heating 105 intercalatedgraphite, such that the heating 105 dramatically changes the physicalproperties of the intercalated graphite. The resultant graphite,referred to herein as expanded graphite (EG) may have significantlyhigher surface area and pore volume that the intercalated graphite.Next, the expanded surface of the EG may be treated with a surfactant tocreating a non-polar bond between the non-polar end of the surfactantand the EG. This leaves the polar end of the surfactant available forlater bonding with the PCM. The compressing 115 compacts the EG tocreate a dense support structure (i.e., a graphite matrix) for the PCM,with surfactant bonded to parts of the EG which are internal to thestructure. The filling 120 bonds the polar end of the surfactant to thePCM, encapsulating the PCM in the graphite matrix.

The exemplary method 100 first includes, heating 105 graphite to formpores which may later be filled 120 with PCM, which may also be referredto as expanding the graphite (because of the expansion of the layers ofthe graphite to form pores). The graphite may be intercalated graphite,which is formed from flake graphite and inserted molecules. Flakegraphite consists of layers of carbon. The carbon is bonded with strongcovalent bonds and the layers are bonded with weak van der Waals bonds.When weak acids are applied to flake graphite, these weak bonds can beovercome and molecules may be inserted (i.e., intercalated) between thelayers. Examples of such acids include sulfuric acid, nitric acid, orchromic acid. FIG. 2 illustrates scanning electron microscope (SEM)images of intercalated graphite, according to some aspects of thepresent disclosure. Layers in the intercalated graphite may be seen inthe highest magnification image, with the inserted molecules shown aswhite edges to the layers. The heating 105 may result in theintercalated graphite layers expanding and/or separating, forming poresor voids within the graphite. FIG. 3 illustrates intercalated graphiteready for heating 105, according to some aspects of the presentdisclosure. After heating 105 the graphite may be referred to asexpanded graphite (EG) due to these pores formed by the separation ofthe layers. FIG. 4 shows EG after being rapidly heated 105, according tosome aspects of the present disclosure. FIG. 5 illustrates SEM images ofthe EG after being heated 105, according to some aspects of the presentdisclosure. The EG may form cylindrical structures when it is heated105.

Heating 105 may include placing the graphite in a furnace or oven orexposing it to a heating element for a period of time. The heating 105may be performed at a temperature in the range between about 200° C. andabout 750° C. A heating element may be an open flame, an induction stovetop, or any other appropriate heating element. The period of time forheating 105 may be in the range between one (1) minute to ten (10)minutes. For example, in some embodiments, the period of time may beapproximately five (5) minutes. The period of time may vary based on thevolume of graphite to be heated 105 (e.g., a longer period of time maybe required for a larger volume of graphite and a shorter period of timemay be required for a smaller volume of graphite) and the time periodmay end when the graphite reaches the desired temperature or when thegraphite expands to the desired porosity. Porosity is the ratio of thetotal volume of the pores of the graphite matrix to the total volume ofthe graphite matrix expressed as a percentage. The total volumeavailable for filling by the PCM is approximately equal to the totalvolume of the pores (as the PCM can only fill the pores), meaning thetotal volume available for filling by the PCM must be less than or equalto the porosity.

The exemplary method 100 shown in FIG. 1 next includes coating 110 thesurface of EG with a surfactant to facilitate better infiltration of thePCM into the graphite matrix during the filling 120. The surfactant maybond to the surface of the EG inside the pores formed during the heating105. The coating 110 may be done by substantially submerging (orsoaking) the EG in a solution containing a surfactant. After successfulfilling of the EG's pores with surfactant, the resultant EG may be driedin a furnace or oven or over a heating element at a relatively lowtemperature (for example, a temperature within the range of about 80° C.to about 150° C.) to remove any excess moisture. The amount ofsurfactant loaded in the wetted graphite may be characterized relativeto the EG. The mass ratio of surfactant applied to the EG may be in therange of about 0.01 to about 0.05. For example, in some embodiments, theratio of surfactant applied to EG may be approximately 0.05.

FIG. 6 illustrates the percent total volume of the graphite matrixfilled with a PCM compared to the porosity of graphite matrix, accordingto some aspects of the present disclosure. Filled in points were filled120 using soaking or submerging, outlined points were filled 120 usingvacuum filtration. The dashed line is representative of the maximumsaturation limit for a given porosity (for example, a graphite matrix of50% porosity can only be filled by PCM to 50% of its total volumebecause only 50% of its total volume is open for filling). Thus, pointsthat have a smaller vertical distance to the dashed line directly aboveit showed better impregnation. Points are shown for two types of PCM:calcium chloride hexahydrate (CaCl₂).6H₂O) (CCH) and disodium phosphatedodecahydrate (Na₂HPO₄.12H₂O) (DHPD), and for 1) no surfactant usedduring the preparation of the PCM composite, 2) applying a surfactant ofoctyl phenol ethoxylate (C₁₄H₂₂O(C₂H₄O)_(n) where n=9-10) (knowncommercially as TX-100) to surface of the graphite matrix after it iscompressed, 3) mixing a surfactant of TX-100 with melted PCM, and 4)coating 110 the surface of the EG with a surfactant of TX-100 prior tocompressing 115 it to form the graphite matrix. There is a cleardelineation between samples coated 110 with a surfactant of TX-100 priorbeing compressed 115 and the other samples. The samples coated 110 witha surfactant prior to being compressed 115 are significantly closer tothe dashed line (indicating more of the available volume filled withPCM) and show that PCM is present in between about 40% to about 80% ofthe total volume of graphite matrix. Potentially higher fill percentagescould be possible with graphite matrices of higher porosity. Soaking andvacuum filtration yield similar results, indicating either may be usedduring filling 120.

Coating 110 may decrease the wetting angle between the salt hydrate meltand the graphite surface, allowing the PCM to infiltrate the pores orvoids of the EG more readily during the filling 120 (as shown in FIG.7). The surfactant may be amphiphilic, having both a hydrophilic end anda hydrophobic end to form a “bridge” between the graphite and the PCM.In some embodiments, the surfactant maybe an anionic surfactant, acationic surfactant, a nonionic surfactant, or an amphoteric(zwitterionic) surfactant. In some embodiments, the nonionic surfactantmay be octyl phenol ethoxylate (C₁₄H₂₂O(C₂H₄O)_(n) where n=9-10)(TX-100) and/or polyethylene glycol tert-octylphenyl ether(C₁₄H₂₂O(C₂H₄O)_(n) where n=7-8) (TX-114). For example, FIG. 8 shows EGbeing submerged in a solution of TX-100, a nonionic surfactant,according to some aspects of the present disclosure.

FIG. 7 illustrates contact angle measurements of a drop of PCM(specifically CCH) on the surface of a graphite matrix A) which had notbeen coated with surfactant, B) which was coated with a surfactant ofTX-100 after being compressed, and C) which had been coated withsurfactant of TX-100 prior to being compressed, according to someaspects of the present disclosure. Contact angle measurements were takenat intervals of 1, 3, and 6 minutes after the PCM drop contacted thesurface of the graphite matrix. As shown in C) coating 110 prior tocompressing 115 resulted in a faster rate of diffusion of PCM (as shownby the faster decrease in contact angle). Despite having a higherinitial contact angle, coating 110 prior to compressing 115 appears tobe more effective at wetting more of the pores (i.e., preparing thepores for infiltration by PCM), including pores which ended up being inthe inner part of the graphite matrix after compression. When thegraphite matrix was compressed 115 prior to being coated 110 (as shownin B)), the surfactant appears to have “spread” across the top surfaceof the graphite matrix but did not infiltrate the inner pores of thegraphite matrix.

As shown by FIGS. 6 and 7, by coating 110 prior to the compressing 115the PCM is able to “attach” to more of the pores within the graphitematrix (via the surfactant). That is, by coating 110 the EG prior tocompressing 115 the EG, the surfactant could wet most surfaces of theEG, including the pores or voids in the interior of the matrix after thecompressing 115.

The exemplary method 100 next includes compressing 115 the EG to form agraphite matrix. Compressing 115 increases the bulk density of the EG(mass divided by the volume occupied) while maintaining the pores.Compressing 115 allows the EG to form a strong support structure for thePCM. In some embodiments, the compressing 115 may be performed using ahydraulic press having a pellet die. The graphite matrix may be in theshape of a disc, a sphere, a brick, or may be substantially planar. Theforce necessary for the compressing 115 may be in the range of about 245N to about 980 N. The final porosity of the graphite matrix may be inthe range of about 60% to about 95%. FIG. 9 shows a compressed graphitematrix made of compressed EG coated in TX-100 in the shape of a disc,according to some aspects of the present disclosure. In the example ofFIG. 9, the compressing 115 was performed using a 40 mm diameter (ID)pellet die.

The porosity (φ) of the matrix after the compressing 115 may bedetermined using the following equation:

$\varphi = {1 - \frac{\rho_{EG}}{\rho_{ED}}}$

where ρ_(EG) is the density of the EG and ρ_(ED) is the effectivedensity of the EG coated with surfactant (i.e., the EG after the coating110). ρ_(ED) may be determined using the following equation:

$\rho_{ED} = {{\frac{m_{EG}}{m_{{EG} + S}} \cdot \rho_{G}} + {\frac{m_{S}}{m_{{EG} + S}} \cdot \rho_{s}}}$

where m_(EG) is the mass of the EG, m_(S) is the mass of surfactant,m_(EG+S) is the combined mass of the EG and the surfactant, ρ_(S) is thedensity of the surfactant, and ρ_(G) is the density of the EG, which maybe assumed to be equivalent to the density of crystal graphite(approximately 2.09 g/cm³). The compressing 115 may be performed at acompression force to achieve the desired porosity. The porosity may bein the range of about 0.6 (60%) to about 0.99 (99%). For example, for amatrix in the shape of a disc with a diameter of 40 mm, to achieve 65%porosity, the compressing 115 may require a force of approximately 1765N. As another example, for a matrix in the shape of a disc with adiameter of 40 mm, to achieve 95% porosity, the compressing 115 mayrequire a force of approximately 40 N.

FIG. 10 illustrates SEM images of graphite matrices (not having beencoated in a surfactant) having A) 62% porosity, B) 73% porosity, C) 83%porosity, and D) 91% porosity, according to some aspects of the presentdisclosure. Notice that the pores become more evident as porosityincreases. The pores having 91% porosity in D) resemble thenon-compressed EG in FIG. 5, while the pores having 62% porosity in A)appear to have been “flattened,” and resembles layers of intercalatedgraphite (as shown in FIG. 3) rather than a collection of pores.

The exemplary method 100 next includes filling 120 the graphite matrixwith a mixture of a PCM and a nucleating agent. In some embodiments, thefilling 120 may be done using soaking and/or vacuum filtration. Forsoaking, the filling 120 may include melting the PCM and the nucleatingagent to form a mixture then submerging (or partially submerging) thegraphite matrix in the mixture of melted PCM and melted nucleatingagent. The submerging may be done while heating the mixture, keeping thePCM and nucleating agent melted during the submerging. For vacuumfiltration, the filling 120 may include placing the mixture and thegraphite matrix, melted PCM, and melted nucleating agent in a vacuumfiltration system. In some embodiments, during the submerging thegraphite matrix and mixture may be in a container which is connected toa vacuum, and any air in the container may be evacuated (i.e., vacuumsoaking or vacuum filtration). The filling 120 may result in the PCMbonding with the surfactant to attach to the graphite matrix. Thefilling 120 may result in the PCM and nucleating agent at leastpartially filling the pores or voids in the matrix. FIG. 11 illustratesgraphite matrix coated in surfactant A) soaking in a mixture of PCM andnucleating agent, and B) undergoing vacuum soaking, according to someaspects of the present disclosure. FIG. 12 illustrates a graphite matrixmade of compressed EG coated TX-100 which has been substantially filledwith a PCM of CCH, according to some aspects of the present disclosure.

In some embodiments, a hydrogel may be incorporated with the PCM andnucleating agent mixture during the filling 120. A hydrogel may extendthe useful life of the PCM composite by preventing loss of water fromthe PCM composite during the hydration and dehydration of the PCM duringusage. For example, when the PCM melts (i.e., dehydration of the PCM),the released water may be absorbed by the hydrogel and later released bythe hydrogel to recombine in the PCM upon freezing/solidification (i.e.,hydration of the PCM). In some embodiments, the hydrogel may be ahydrophilic polymer such as poly(acrylamide-co-acrylic acid) (PAAAM),poly(acrylamide-co-sodium acrylate) (PAASA), and/or alginate.

In some embodiments, a hydrogel may be used in place of the graphitematrix for the PCM composite providing substantially total encapsulationof the PCM and nucleating agent by the hydrogel. In some embodiments,the PCM/hydrogel encapsulation may be laced with thermally conductivefillers or encapsulated in a 3D printed thermally conductive polymerfoam scaffold for enhancing the thermal conductivity. For example,ultrahigh molecular weight polyethylene (UHMWPE) could be used as athermally conducting filler and/or three-dimensional (3D) printedthermally conductive polymer foams could be used as thermally conductiveporous scaffolds. Encapsulation of the PCM by a hydrogel may eliminatephase separation, ensuring form-stability and long cycle-life, promoteheterogeneous nucleation reducing supercooling, and may prevent leakageof liquid PCM. In some embodiments, the hydrogel encapsulated PCMcomposites may contain 5-10 weight percent (wt %) hydrogel, 2-3 wt %thermally conductive fillers/scaffolds, 2-3% nucleating agents, and85-90 wt % salt hydrates.

In some embodiments, a hydrogel as encapsulation may be used to containPCM using a core shell structure. This may eliminate the phaseseparation problem common in inorganic salt hydrates. The hydrogel maymodulate the amount of water being released and absorbed by the PCM whenundergoing a phase transition. The amount of water may be stored in thehydrogel and will be utilized when needed. The PCM may under anenergetic transition release or absorb heat. PCM may be of greatinterest for thermal energy storage in building applications, wheresub-cooling and phase separation often limit the use of PCMs. In someembodiments, using a hydrogel or other scaffolding may eliminate thephase separation and may address the sub-cooling by mixing nucleatingagents with the PCM before filling a hydrogel. On the external wall ofthe hydrogel, a hydrophobic layer may be created to prevent the waterfrom permeating out of the hydrogel.

By coating 110 prior to the compressing 115, the methods describedherein may result in greater infiltration of PCM into the graphitematrix. FIG. 13 illustrates SEM images of A) a graphite matrix coatedwith surfactant after being compressed and having a porosity of 84%, andB) a graphite matrix coated with surfactant prior to being compressedand having a porosity of 83%, according to some aspects of the presentdisclosure. In both A) and B), the visible edges of the graphite layersappear to be “glowing”, more so in A) where the EG was compressed priorto being coated 110. The images in A) closely resemble the images ofFIG. 5, where no surfactant was present. This indicates that compressing115 prior to coating 110 does not do as well at infiltrating the poresof the graphite matrix as coating 110 prior to the compressing 115 (asshown in B)).

The amount of nucleating agent used in the filling 120 may expressed asa concentration in the solution with the PCM. The weight percent ofnucleating agent to PCM may be in the range of about 0.1 wt % to about 5wt %. In some embodiments, the weight percent of nucleating agent in thesolution may be less than 3 wt %. The remaining weight percent mayconsist mostly of PCM.

The PCM may be eutectic, organic, or inorganic. Eutectic PCMs may beorganic-organic, inorganic-organic, or inorganic-inorganic. Organic PCMsmay be paraffin or non-paraffin compounds. Organic PCMs generally do notsuffer from significant supercooling or phase separation. Inorganic PCMsmay be salt hydrates or metallics. Salt hydrates are alloys of inorganicsalts (AB) and water (H₂O) resulting in a typical crystalline solid ofgeneral formula (AB.xH₂O). Exemplary salt hydrates include calciumchloride hexahydrate (CaCl₂).6H₂O) (CCH), calcium bromide hexahydrate(CaBr₂.6H₂O), disodium phosphate dodecahydrate (Na₂HPO₄.12H₂O) (DHPD),disodium sulfate decahydrate (Na₂SO₄.10H₂O), zinc nitrate hexahydrate(Zn(NO₃)₂.6H₂O), magnesium chloride hexahydrate (MgCl₂.6H₂O), magnesiumnitrate hexahydrate (Mg(NO₃)₂.6H₂O), and lithium nitrate trihydrate(LiNO₃.2H₂O).

FIG. 14 illustrates SEM images of a graphite matrix substantially filledwith A) a PCM of calcium chloride hexahydrate (CaCl₂).6H₂O) (CCH) bysoaking, B) a PCM of CCH by pre-compression vacuum filtration, C) a PCMof disodium phosphate dodecahydrate (Na₂HPO₄.12H₂O) (DHPD) by soaking,and D) a PCM of DHPD by pre-compression vacuum filtration, according tosome aspects of the present disclosure. Submerging the graphite matrixin the PCM with or without vacuuming show a coating of PCM throughoutthe pores and surfaces of the graphite matrix, indicating effectivefilling 120. The DHPD samples in C) and D) display some sphericalcrystals throughout the graphite matrix, most likely due to theparticular crystal structure of DHPD when compared to the structure ofCCH.

FIG. 15 illustrates the relationship between the PCM:EG mass ratio andthe bulk density, according to some aspects of the present disclosure.Filled icons indicate a PCM of CCH, outlined icons indicate a PCM ofDHPD. Bulk density was calculated using the volume of the disk and thefinal mass after filling 120, as shown below:

$\rho = \frac{{final}\mspace{14mu}{mass}}{{thickness} \times r^{2} \times \pi}$

Points with a higher bulk density and a higher PCM:EG mass ratio showmore filling 120 of the graphite matrix by the PCM. Both higher bulkdensity and higher PCM:EG mass ratio indicate that there is a highamount of PCM present in the graphite matrix. As both soaking/submergingand pre-compression vacuum filtration (i.e., filling 120 using vacuumfiltration prior to the compression 115) show comparable bulk densityvalues, but the latter appears to consistently show a significantlyhigher PCM:EG mass ratio. This indicates that more PCM per mass of EG ispresent in the samples (all samples had the same mass of EG). Therefore,pre-compression vacuum filtration may be a more effective means offilling 120 than soaking/submerging.

FIG. 16 illustrates the PCM accumulation within the graphite matrix overtime, according to some aspects of the present disclosure. As shown inFIG. 16, all of the samples approached their maximum saturation pointafter only 20 minutes, and pre-compression surfactant treatment of theEG clearly has a significant positive effect on PCM accumulation. Notethat soaking time of pre-compression samples was minimized due to thedisk losing structural integrity as it becomes more saturated with PCM.The saturation trends shown for these samples indicate that at the timethe soaking a been stopped, there was probably more empty pores to befilled; however, such results were not obtained due to the disks fallingapart after only about an hour of soaking.

FIG. 17 illustrates the relationship between heat flow and temperaturefor a mixture of PCM of CCH and 1, 2, and 5% SCH (subjected to fourcycles), according to some aspects of the present disclosure. Note thatdue to the small sample size and sensitivity of the instrument,differential scanning calorimeter (DSC) data often exaggeratessupercooling; however, all samples showed comparable and consistentmelting curves.

Table 1 shows average values for heat of fusion, heat of freezing,melting temperature, and freezing temperature for the samples in FIG. 17over four cycles. Any given PCM composite has lower enthalpy values thanpure PCM due to the presence of graphite (in the form of the graphitematrix) in the sample. Note that 5.0% SCH shows the best combination ofretained enthalpy values and improved supercooling compared to the pureCCH sample: it has neither the highest enthalpy values nor largestreduction in supercooling of all samples, but when considering bothfactors jointly, 5.0 wt % SCH shows the most ideal compromise.

Table 1 shows average values for heat of fusion, heat of freezing,melting temperature, and freezing temperature for the samples in FIG. 17over four cycles.

Average Average Average Average Melting Freezing Heat of Heat ofTemperature Temperature Sample Fusion (J/g) Freezing (J/g) (° C.) (° C.)Pure CCH 162.7 159.92 27.25 0.67 CCH w/1.0 134.63 64.95 29.54 2.30 wt %SCH CCH w/2.0 164.63 127.03 29.30 0.74 wt % SCH CCH w/5.0 161.6 107.4729.85 1.61 wt % SCH

Based on the temperature-history results shown in FIG. 18, the additionof EG improves supercooling for both CCH and DHPD. Additionally, thepresence of SCH in CCH samples also significantly improves supercooling;these results are highlighted in FIG. 19, which similar data as FIG. 18but with selected CCH samples only. Table 2 summarizes the nucleationand phase change temperatures of samples in FIG. 19, indicating thedegree of supercooling in each sample. In general, supercooling wasreduced from 30° C. in the pure CCH sample to about 3° C. in theEG/CCH/SCH 5.0 wt %.

FIG. 18 illustrates cooling curves of pure PCM and graphite matricesfilled with PCM, over four cycles, according to some aspects of thepresent disclosure. The four examples shown in FIG. 18 show coolingcurves of pure PCM, CCH with varying concentrations of SCH filled intoEG, and DHPD filled into EG. The negative peaks in the cooling curvesdepict the degree of supercooling in each sample. Note that in cycle 3(bottom left of FIG. 18) the pure CCH never freezes: this illuminatesthe randomness and unpredictability of nucleation.

FIG. 19 illustrates cooling curves of pure PCM and PCM composites overfour cycled, according to some aspects of the present disclosure. Thecooling curves shown in FIG. 19 are the same as the cooling curves asshown in FIG. 18, but with only pure CCH, CCH filled into EG, and CCHwith 5 wt % SCH filled into EG to highlight the effectiveness of bothmethods in nucleation. While EG itself significantly improvessupercooling

FIG. 20 illustrates enthalpy vs temperature curves for PCMs of A) CCHand B) DHPD for several cycles. The presence of EG in the samples (solidlines) does not significantly reduce the amount of latent heat,especially in the case of CCH. Table 2 shows the nucleation temperaturesand phase change temperatures for three types of the samples shown inFIG. 20 over four cycles. Nucleation temperature corresponds to thenegative peak in the cooling curve, and the phase change temperaturecorresponds to the horizontal plateau in each respective sample. PureCCH cycle 3 has no values because that sample did not crystalize duringthis cycle.

TABLE 2 Nucleation and Phase Change Temperatures for Samples. NucleationPhase Change Sample Cycle Temperature (° C.) Temperature (° C.) Pure CCH1 2.58 29.48 2 0 29.32 3 — — 4 0 29.44 EG/CCH 1 16.78 26.62 2 20.6826.37 3 20.51 26.27 4 17.12 26.45 EG/CCH/SCH 1 24.61 27.75 5 wt % 224.92 27.74 3 25.04 27.67 4 24.87 27.75

Supercooling may occur when the PCM remains in the liquid phase when itis below its phase transition temperature (i.e., the temperature when ittransitions from a liquid to a solid). A nucleating agent may reducesupercooling by providing a structure for the PCM to begin nucleation(i.e., encouraging the PCM to form a solid) when it is below its phasetransition temperature. The phase transition temperature of thenucleating agent should be higher than the PCM so that the nucleatingagent may remain substantially solid when the PCM is in the liquidstate.

In some embodiments, the nucleating agent may have a substantiallyisomorphous lattice structure to the lattice structure of the PCM. Thatis, the lattice parameters of the nucleating agent may be within about15% of the lattice parameters of the PCM. For example, calcium chloridehexahydrate (CaCl₂).6H₂O), a salt hydrate PCM has a hexagonal latticestructure and a melting point of approximately 30° C.; an appropriatenucleating agent for such a PCM may be strontium chloride hexahydrate(SrCl₂.6H₂O) which also has a hexagonal lattice structure and a meltingpoint of approximately 115° C. Other exemplary nucleating agents whichare substantially isomorphous to some salt hydrate PCMs includestrontium bromide hexahydrate (SrBr₂.6H₂O), strontium iodide hexahydrate(SrI₂.6H₂O), barium iodide hexahydrate (BaI₂.6H₂O), barium chloridehexahydrate (BaCl₂.6H₂O), barium chloride (BaCl₂), barium bromidedihydrate (BaBr₂.2H₂O), barium bromide (BaBr₂), barium carbonate(BaCO₃), strontium carbonate (SrCO₃), barium fluoride (BaF₂), strontiumfluoride (SrF₂), and barium hydrofluoride (Ba(HF₂)). For example, FIG.11 shows a graphite matrix submerged in a mixture of melted calciumchloride hexahydrate (CaCl₂).6H₂O) (a salt hydrate PCM) and strontiumchloride hexahydrate (SrCl₂.6H₂O) (a lattice-matched nucleating agent),according to some embodiments of the present disclosure. FIG. 12illustrates a final PCM composite of a surface-treated compressedgraphite matrix containing calcium chloride hexahydrate (CaCl₂).6H₂O) (asalt hydrate PCM) and strontium chloride hexahydrate (SrCl₂.6H₂O) (alattice-matched nucleating agent), according to some embodiments of thepresent disclosure.

In some embodiments, the nucleating agent may have a substantiallynon-isomorphous lattice structure to the lattice structure of the PCM.That is, the lattice parameters of the nucleating agent may not bewithin about 15% of the lattice parameters of the PCM. Exemplarynucleating agents which are substantially non-isomorphous to some salthydrate PCMs include barium oxide (BaO), barium hydroxide (Ba(OH)₂)),barium sulfate (BaSO₄), barium carbonate (BaCO₃), and strontiumhydroxide (Sr(OH)₂).

The foregoing discussion and examples have been presented for purposesof illustration and description. The foregoing is not intended to limitthe aspects, embodiments, or configurations to the form or formsdisclosed herein. In the foregoing Detailed Description for example,various features of the aspects, embodiments, or configurations aregrouped together in one or more embodiments, configurations, or aspectsfor the purpose of streamlining the disclosure. The features of theaspects, embodiments, or configurations may be combined in alternateaspects, embodiments, or configurations other than those discussedabove. This method of disclosure is not to be interpreted as reflectingan intention that the aspects, embodiments, or configurations requiremore features than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment, configuration, oraspect. While certain aspects of conventional technology have beendiscussed to facilitate disclosure of some embodiments of the presentdisclosure, the Applicants in no way disclaim these technical aspects,and it is contemplated that the claimed invention may encompass one ormore of the conventional technical aspects discussed herein. Thus, thefollowing claims are hereby incorporated into this Detailed Description,with each claim standing on its own as a separate aspect, embodiment, orconfiguration.

1. A composition comprising: a graphite matrix comprising an expandedgraphite having a plurality of pores defining a pore volume; asurfactant having a first end and a second end; a mixture comprising aphase change material and a nucleating agent positioned within betweenabout 40% and about 95% of the pore volume; wherein: the first end ofthe surfactant is bonded to the expanded graphite, the second end of thesurfactant is bonded to the phase change material, the surfactant ispresent in a mass ratio of the surfactant to the expanded graphitebetween about 1:100 and about 5:100, and the nucleating agent is presentin the mixture at a concentration between greater than zero weightpercent (wt %) and less than about 6.0 wt %.
 2. The composition of claim1, wherein the pore volume is between about 60% and about 95% of a totalvolume of the graphite matrix as defined by the pore volume plus avolume of the expanded graphite present in the graphite matrix.
 3. Thecomposition of claim 1, wherein the surfactant comprises at least one ofoctyl phenol ethoxylate (C₁₄H₂₂O(C₂H₄O)_(n) where n=9-10) (TX-100) orpolyethylene glycol tert-octylphenyl ether (C₁₄H₂₂O(C₂H₄O)_(n) wheren=7-8) (TX-105).
 4. The composition of claim 1, wherein the mixturefurther comprises a hydrogel comprising at least one ofpoly(acrylamide-co-acrylic acid) (PAAAM), poly(acrylamide-co-sodiumacrylate), or alginate.
 5. The composition of claim 1, wherein the phasechange material comprises a salt hydrate.
 6. The composition of claim 5,wherein the salt hydrate comprises at least one of calcium chloridehexahydrate (CaCl₂).6H₂O), calcium bromide hexahydrate (CaBr₂.6H₂O),disodium sulfate decahydrate (Na₂SO₄.10H₂O), disodium phosphatedodecahydrate (Na₂HPO₄.12H₂O), zinc nitrate hexahydrate (Zn(NO₃)₂.6H₂O),magnesium chloride hexahydrate (MgCl₂.6H₂O), magnesium nitratehexahydrate (Mg(NO₃)₂.6H₂O), or lithium nitrate trihydrate (LiNO₃.2H₂O).7. The composition of claim 5, wherein the nucleating agent comprises atleast one of strontium chloride hexahydrate (SrCl₂.6H₂O), strontiumbromide hexahydrate (SrBr₂.6H₂O), strontium iodide hexahydrate(SrI₂.6H₂O), barium iodide hexahydrate (BaI₂.6H₂O), barium chloridehexahydrate (BaCl₂.6H₂O), barium chloride (BaCl₂), barium carbonate(BaCO₃), strontium carbonate (SrCO₃), barium fluoride (BaF₂), strontiumfluoride (SrF₂), barium hydrofluoride (Ba(HF₂)), barium oxide (BaO),barium hydroxide (Ba(OH)₂)), barium sulfate (BaSO₄), or strontiumhydroxide (Sr(OH)₂).
 8. A method comprising, in order: heating anintercalated graphite to a temperature between about 200° C. and about750° C. resulting in an expanded graphite; coating the expanded graphitewith a surfactant having a first end and a second end to form a wettedgraphite; compressing the wetted graphite to form a graphite matrixhaving a plurality of pores defining a pore volume; and filling betweenabout 40% and about 95% of the pore volume with a mixture comprising aphase change material and a nucleating agent resulting in an energystorage material; wherein: the surfactant is present in a mass ratio ofthe surfactant to the expanded graphite between about 1:100 and about5:100, the first end of the surfactant is bonded to the expandedgraphite and the second end of the surfactant is bonded to the phasechange material, and the nucleating agent is present in the mixture at aconcentration between greater than zero wt % and less than about 6.0 wt%.
 9. The method of claim 8, wherein the pore volume is between about60% and about 95% of a total volume of the graphite matrix as defined bythe pore volume plus a volume of the expanded graphite present in thegraphite matrix.
 10. The method of claim 8, wherein the heatingcomprises: placing the intercalated graphite in a furnace for a periodof time, wherein: the furnace is operated at a temperature of betweenabout 200° C. and 750° C.
 11. The method of claim 10, wherein the periodof time is between about one (1) minute and about ten (10) minutes. 12.The method of claim 8, wherein the coating comprises: submerging theexpanded graphite in a solution comprising the surfactant.
 13. Themethod of claim 8, wherein the wherein the surfactant comprises at leastone of octyl phenol ethoxylate (C₁₄H₂₂O(C₂H₄O)_(n) where n=9-10) orpolyethylene glycol tert-octylphenyl ether (C₁₄H₂₂O(C₂H₄O)_(n) wheren=7-8).
 14. The method of claim 8, wherein the compressing comprises:placing the wetted graphite in a hydraulic press having a pellet die;and pressing the pellet die on the wetted graphite resulting in thegraphite matrix.
 15. The method of claim 8, wherein the fillingcomprises: melting the phase change material and the nucleating agent toform the mixture; and submerging the graphite matrix in the mixtureresulting in a slurry.
 16. The method of claim 15, wherein the fillingfurther comprises: performing vacuum filtration on the slurry during thesubmerging, resulting in the energy storage material.
 17. The method ofclaim 8, wherein: a hydrogel comprising at least one ofpoly(acrylamide-co-acrylic acid) (PAAAM), poly(acrylamide-co-sodiumacrylate), or alginate is added to the mixture prior to the filling 18.The method of claim 8, wherein the phase change material comprises asalt hydrate.
 19. The method of claim 18, wherein the salt hydratecomprises at least one of calcium chloride hexahydrate (CaCl₂).6H₂O),calcium bromide hexahydrate (CaBr₂.6H₂O), disodium sulfate decahydrate(Na₂SO₄.10H₂O), disodium phosphate dodecahydrate (Na₂HPO₄.12H₂O), zincnitrate hexahydrate (Zn(NO₃)₂.6H₂O), magnesium chloride hexahydrate(MgCl₂.6H₂O), magnesium nitrate hexahydrate (Mg(NO₃)₂.6H₂O), or lithiumnitrate trihydrate (LiNO₃.2H₂O).
 20. The method of claim 18, wherein thenucleating agent comprises at least one of strontium chloridehexahydrate (SrCl₂.6H₂O), strontium bromide hexahydrate (SrBr₂.6H₂O),strontium iodide hexahydrate (SrI₂.6H₂O), barium iodide hexahydrate(BaI₂.6H₂O), barium chloride hexahydrate (BaCl₂.6H₂O), barium chloride(BaCl₂), barium carbonate (BaCO₃), strontium carbonate (SrCO₃), bariumfluoride (BaF₂), strontium fluoride (SrF₂), barium hydrofluoride(Ba(HF₂)), barium oxide (BaO), barium hydroxide (Ba(OH)₂)), bariumsulfate (BaSO₄), or strontium hydroxide (Sr(OH)₂).